Nitrogen oxides contained in exhaust gases discharged from factories, incineration facilities and the like are strictly regulated on the amount thereof discharged because these have an adverse effect on the human body and additionally work out to a substance causing acid rain. The nitrogen oxide generally called NOx which has been heretofore a target of the exhaust regulation, includes nitrogen monoxide (NO) and nitrogen dioxide (NO2). For eliminating these nitrogen oxides, a catalytic reduction process using a reducing substance, or a ternary catalyst process is already put into practical use. In recent years, a process of allowing hydrocarbon to coexist in the presence of a zeolite or alumina catalyst has been proposed.
Out of nitrogen oxides, nitrous oxide has been heretofore not regulated on the exhaust value and released into atmosphere without passing through any decomposition treatment. However, at the International Global Warming Conference (COP3), nitrous oxide is, as well as nitrogen dioxide, methane and flon, particularly taken notice of as a global pollutant which brings about elevation of temperature due to greenhouse effect (the warming effect is as high as about 300 times the carbon dioxide) and the concern about the reduction in the release of nitrous oxide into atmosphere is increasingly growing.
Under these circumstances, means for removing nitrous oxide contained in exhaust gas is being aggressively studied and several methods have been proposed. For example, for removing nitrous oxide, a catalytic reduction process of reacting the exhaust gas with a catalyst in the presence together of a reducing gas (see, JP-A-2-068120 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)) and a catalyst comprising zeolite-type support having supported thereon a transition metal (see, JP-A-4-363143) are known. These means have, however, a problem in that the treatment temperature is high or when moisture is present in the gas treated, the catalyst is deactivated.
In the medical field, contamination of an operating room by anesthetic gas and adverse effect on the health of workers have been taken as a matter of issue since 1960. It is known that as a result of long term inhalation of the anesthetic gas (a mixed gas containing nitrous oxide and a volatile anesthetic) leaked in the operating room, the health is disordered. In the U.S.A., the National Institute for Occupational Safety and Health (NIOSH) recommends to reduce, as a permissible standard, nitrous oxide (N2O) to 25 ppm or less and a volatile anesthetic to 2 ppm in the case of a sole use and to 0.5 ppm or less in the case of a combination use with nitrous oxide. Accordingly, all anesthesia machines are obliged to be equipped with a waste anesthetic gas removing apparatus and at the present time, the environment in the operating room can reach the above-described levels.
The waste anesthetic gas removing apparatus is an apparatus for discharging the waste anesthetic gas outdoors from the exhalation of a patient by letting a compression air or the like to accompany the gas. However, the gas discharged from each operating room by the waste anesthetic gas removing apparatus is released into atmosphere without passing through any treatment at the present time. This technique may improve the environment within the operating room but is disadvantageous in view of the environmental issue of global warming from the reasons described above and the anesthetic gas is demanded to remove or be rendered harmless before the release into atmosphere.
Known examples of the catalyst capable of decomposing nitrous oxide in the waste anesthetic gas include:
(1) a catalyst mainly comprising at least one member selected from the group consisting of platinum, palladium, rhodium, iridium and ruthenium (see, JP-B-61-045486 (the term “JP-B” as used herein means an “examined Japanese patent publication”));
(2) a catalyst containing an iron family metal and an oxide of a rare earth element or a catalyst having further added thereto at least one platinum family (see, JP-B-61-45487);
(3) a catalyst mainly comprising a mixture of cupric oxide and chromium oxide or a catalyst having further added thereto at least one member selected from the group consisting of ferric oxide, nickel oxide, cobalt oxide and manganese dioxide (see, U.S. Pat. No. 4,259,303); and
(4) a catalyst mainly comprising at least one of ferric oxide and chromium oxide (see, U.S. Pat. No. 4,259,303).
However, according to the process for decomposing nitrous oxide using the catalyst described in (2), (3) and (4) above, nitrous oxide in a high concentration may be decomposed but nitrogen monoxide (NO) and nitrogen dioxide (NO2) (hereinafter sometimes collectively referred to as “NOx”) as nitrogen oxides are produced in an amount of 5 to 32 ppm, thus, generation of NOx in excess of the allowable concentration of 3 ppm (TWA, time weighted average) for NO2 remains as a problem. According to the process for decomposing nitrous oxide using the catalyst described in (1), when moisture in an amount of, for example, approximately from 1 to 3% is present in the reaction gas, the catalyst may decrease in the activity, and this remains as a problem to be solved.
The present invention has been made under these circumstances and the object of the present invention is to provide a catalyst for decomposing nitrous oxide, which is not easily deteriorated due to moisture, has low-temperature decomposition activity and can reduce the amount of NOx generated to lower than the allowable concentration. The object of the present invention includes providing a process for producing the catalyst and a method for decomposing nitrous oxide.